1. Field of the Invention
The present invention relates to a wireless communication system, and more specifically, to an OFDM system for wireless communication in a wireless local area network.
2. Description of the Prior Art
Wireless local area networks (WLANs) are ever increasingly being used in network environments where mobility is of importance. Orthogonal frequency division multiplexing (OFDM) is a well-known concept used in implementing WLAN hardware. A typical WLAN employing OFDM can achieve a maximum data transfer rate of 54 Mbps per client, which is significantly less than the wire-based LAN capability of between 100 Mbps to 10 Gbps. This 54 Mbps transfer limit for WLANs is a consequence of current technological limitations and regulation, such as that according to IEEE 802.11a or 802.11g for example. For conventional WLANs, the advantage of mobility can be enhanced by an improvement in data rate.
FIG. 1 illustrates a convention WLAN 10 including an access point 12, a first user terminal 14, and a second user terminal 16. The WLAN 10 is very much typical of an IEEE 802.11a or 802.11g implementation. The access point 12 includes four antennas (or antenna pairs) for communicating data with the terminals 14, 16, the first user terminal 14 having a single antenna and the second user terminal 16 having two antennas. In the access point 12, a single antenna is used to communicate with the first user terminal 14, and two antennas are used to communicate with the second user terminal 16 over three frequency bands in total.
FIG. 2 illustrates the frequency band assignment of the WLAN 10 of FIG. 1. As each antenna operates in a distinct frequency band, the first user terminal 14 uses a first frequency band, while the second user terminal 16 uses second and third frequency bands. Thus, in accordance with the above-mentioned 54 Mbps transfer rate limitation, the first user terminal 14 and second user terminal 16 have maximum data rates of 54 Mbps and 108 Mbps respectively. Increasing these data rates can only be facilitated by increasing the number of antennas in the user terminals 14, 16 and consequently increasing the number of available frequency bands. In addition, if the WLAN 10 has only three frequency bands available for use, the access point 12 is encumbered with an extra antenna that cannot be used to communicate with another user terminal.
Frequency band assignments for WLANs are set forth in IEEE standards 802.11a and 802.11g, for example. According to IEEE Std 802.11a-1999, the 5 Ghz band comprises 12 frequency bands for data communication. Similarly, the 2.4 Ghz band of IEEE 802.11g offers three frequency bands. Following these specifications, prior art implementations have been constrained to one band per antenna and the resulting 54 Mbps maximum data rate per band.
Therein lies the main problem with the prior art regarding transfer rate. Specifically, in order to increase the data rate to a given terminal, more antennas and thus more frequency bands (available frequency bands being limited in number) must be employed. This runs counter to the need to free up frequency bands for communication with a larger number of terminals. In short, the prior art suffers from limitations in data rate per frequency band.